NUTRIENT MEASUREMENTS IN THE INDIAN AND PACIFIC OCEAN

Objective:
Determination of dissolved nutrients (phosphate, silicate, nitrate
and nitrite) in the ocean as a contribution to the OACES, WOCE, JGOFS
and GLOBEC programs.
Rationale:
The distribution of inorganic nutrients (phosphate, silicate,
nitrate and nitrite) in the ocean is controlled by physical, biological
and geochemical processes. Biological processes also link nutrient and
CO2 cycles in the marine environment. In fact, the availability of
inorganic nutrients in the euphotic zone is an important factor
controlling primary productivity and the efficiency of the so-called
biological carbon pump (and for that matter the energy available for
secondary productivity). During regeneration, bacterial oxidation of
dissolved and sinking organic matter yields CO2 and inorganic phosphate
and nitrate. Nutrient concentrations corrected for bacterial remineralization
can be used as conservative tracers in the study of ocean
circulation as various water masses have distinctive distributions of
"preformed" nutrients.

The major sampling effort in the Indian Ocean was focused along the I8N line.
This is a repeat WOCE hydrographic line and also the sole mid-ocean meridional WOCE
cruise during the southwest monsoon season. The depth and
meridional extent of monsoonal upwelling can be be used to estimate the
monsoonal upwelling nutrient flux for the central Indian Ocean.
Nutrient data will aid in examining monsoonal effects on upper ocean
circulation in the tropics, determining the seasonal variability of
equatorial currents as well as the Southern Indian Subtropical Gyre,
and in elucidating the deep water circulation in the central Indian Ocean.

Nutrient data collected along line P15S will be used to
identify water masses within the thermocline as well as the intermediate and
bottom waters flowing northward into the Southwest Pacific Basin and
through the Samoa Passage. The nutrient data will also be used to estimate
nutrient budgets for the Southwest Pacific Basin.

Method:
An Alpkem Flow Solution System and a Technicon Autoanalyzer II
were used as analytical instruments. Nutrient measurements on the
oceanic cruises closely followed WOCE protocols for chemical
analysis, standardization and data analysis (Gordon et al. 1991).
Some modifications in the chemical analysis were required to optimize
the Flow Solutions system.
Accomplishment:
Nutrient analyses has been successfully completed to required
specifications of precision and accuracy on the NOAA sponsored WOCE
line I8N in the Indian Ocean along 80 degrees E (Sept.-Oct. 1995)
and P15S in the Pascific Ocean along 170 degrees W (Jan.-Mar. 1996).
Samples were analyzed for silicic acid, phosphate, nitrate and nitrite
on an autoanalyzer in a temperature controlled van, usisng procedures
similar to those specified in the WOCE nutrient protocol (Gordon et al., 1991).
A total of 101 stations along I8N and 182 stations along P15S were
occupied, resulting in about 10,000 and 24,000 nutrient data values
along the I8N and P15S lines, respectively. Preliminary data was provided daily or
weekly during the cruise, and a nearly complete preliminary data set
was provided to the chief scientists before disembarking the ship. Some
comparisons of our preliminary data with data from another recent
WOCE cruise at a nearby site are shown in
Figures1,2, 3. Assembly, processing and quality control of nutrient
data collected along I8N and P15S is underway.
Key reference:
Gordon, L.I., Joe C. Jennings, Jr., Andrew A. Ross, and James M. Krest.
1992. A suggested protocol for continuous flow automated
analysis of seawater nutrients (Phosphate, Nitrate, Nitrite
and Silicic Acid) in the WOCE Hydrographic Program and the
Joint Global Ocean Fluxes Study. OSU Coll. of Oc. Descr.
Chem. Oc. Grp. Tech. Rpt. 92-1.
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